1. Field of the Invention
The present invention relates to an image reading apparatus, a driving method of the image reading apparatus, a storage medium and a program, and more particularly to the technique suitable for use with an image reading apparatus, which uses a CCD as an image pickup element and can selectively change a reading resolution.
2. Related Background Art
FIG. 6 is a cross sectional view showing an example of the whole structure of a copying machine constituted of an image reading unit (image reading apparatus) 136 and an image forming apparatus unit (image forming apparatus) 100.
As shown in FIG. 6, the image forming apparatus unit 100 is a removable cartridge in which a photosensitive drum 110, a roller charger 134, and a cleaner box 133 for accumulating waste toner, toner and a toner container are integrated. The cartridge is replaced by a new one when the number of printed sheets reaches a predetermined number. Above the image forming unit 100, a laser diode 104 constituting an exposure unit, a polygon mirror 105 to be rotated by a high speed motor not shown, lenses 106 and 107 and a turnback mirror 108 are arranged. The roller charger 134 is supplied with an A.C. voltage at a frequency of several hundreds Hz superposed upon a D.C. voltage, and the photosensitive drum 110 is uniformly charged by the roller charger 134.
The laser diode 104 emits a laser beam in response to image information (image signal) supplied from the image reading unit 136 or an external apparatus such as a personal computer. The output laser beam is applied to the photosensitive drum 110 via an optical path (polygon mirror 105, lenses 106 and 107 and turnback mirror 108). As the photosensitive drum 110 rotates in the arrow direction shown in FIG. 6, an image corresponding to the image information is visualized with developing toner coated by a developer sleeve 109.
The toner image on the photosensitive drum 110 is transferred to a transfer sheet 150 by a transfer roller 135 in accordance with a voltage across the photosensitive drum 110 and transfer roller 135 supplied from a high voltage source not shown. This transfer sheet 150 is supplied from a cassette 131 housed in the image forming unit 100 or a cassette 132 on a sheet feeding deck 101 (an optional unit for newly installing a cassette other than the cassette 131).
The transfer sheet 150 onto which the toner image is transferred is transported by a transport roller 116 to a well-known fixing unit 111 for heating and pressing the transfer sheet. The transferred toner still not fixed is melted and fixed by the fixing unit 111 to be ejected out on a face-down tray (image directed downward) 113. In this manner, an image corresponding to the supplied image information can be obtained. Toner not transferred to the transfer sheet 150 by the transfer roller 135 and left on the photosensitive drum 110 is removed through cleaning by a well-known fur brush, blade or the like.
The image reading unit 136 acquires image data of an original image. In the image reading unit 136, lamps 137 and 138 apply light to an original. Reflected light is focused on CCDs 139 and 140 by using an optical system such as a lens and mirrors to photoelectrically convert an optical image into digital image data of each line. A reading unit in which the lamps 137 and 138, the optical system such as a lens and mirrors and CCDs 139 and 140 are integrated is driven along a sub-scan direction (direction perpendicular to the line) to read the image of one page and obtain digital image data of the original image. Reference numeral 141 denotes a carriage for moving the reading unit in parallel to the original base while always maintaining a constant distance from the original surface.
FIG. 7 is a block diagram showing the conventional structure of CCD and its peripheral circuits arranged in the carriage 141.
Referring to FIG. 7, light irradiated from the lamp 138 and then reflected from the original 142 is focused on CCD 140 via a mirror 143 and a lens 144. A timing generation circuit 145 generates drive signals (CCD drive signals ø1, ø2, SH and RS) at predetermined timings and supplies them to a CCD driving circuit 146. In response to the CCD drive signals ø1, ø2, SH and RS, the CCD driving circuit 146 drives CCD 140. CCD 140 outputs an image signal OS obtained through photoelectric conversion of light reflected from the original 142.
FIG. 8 is a timing chart of CCD driving.
In FIG. 8, ø1 and ø2 are clock signals used for transferring charges accumulated in accordance with incident light (reflected light) in photodiode array arranged in an in CCD. RS is a reset pulse used for injecting charges transferred by the clock signals ø1 and ø2 into a floating capacitor provided in an output unit of the CCD. OS is an image signal that is output from a source follower constituted of MOS transistors, in accordance with a potential changed with the charges injected into the floating capacitor.
As shown in FIG. 8, charges accumulated in each photodiode of CCD are sequentially transferred by using the clock signals ø1 and ø2 and injected into the floating capacitor in response to the reset pulse RS. A change in the potential caused by the injection of charges into the floating capacitor is output as the image signal OS from the source follower.
In the conventional image reading apparatus described above, the drive signals for driving functional units of the image reading apparatus may adversely affect the image signal depending upon the generation (output) timings (a timing signal generating method) of the drive signals, because of a variation in characteristics of floating capacitors of CCDs caused by a manufacture variation. Therefore, moire may be formed in a formed image depending upon a half-tone input pattern of the original image. Namely, in the state that CCD operates at highest speed, drive signals influence the output image signal in some case depending upon the transfer speed and floating capacitor.
For example, as shown in FIG. 9, if CCD is driven at a highest operation speed with clock signals, in order to improve an SN ratio it is necessary to set the timing for sampling the image signal OS near at the trailing edge of one period of a transfer clock signal in order to hold a peak level of the image signal. The reset pulse RS for injecting charges transferred to the CCD output into the floating capacitor is also required to be set near at the trailing edge of one period of the clock signal.
In the conventional image reading apparatus, however, the drive signals are generated at fixed timings so that the set-up time and hold time of each drive signal or the like is very critical. If there is a variation in characteristics of floating capacitors caused by a manufacture variation, crosstalks or the like to the image signal OS occur at the timing when the reset pulse RS is enabled and a sampling pulse CP of the image signal OS is disabled, which may result in an inability to read an image correctly.